<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    div{
        width: 300px;
        background-color: rgba(0,0,0,0.3);
        padding: 10px;
        position: relative;/*做参照物*/
    }
    input{
        border: none;/*去掉边框*/
        width: 260px;
        padding: 10px 20px;

    }
    img{
        position: absolute;
        right: 30px;
        top: 14px;
    }
    p{
        font-size: 12px;
        color: red;
        margin: 0;
    }
  </style>
</head>
<body>
<div>
  <input type="text" placeholder="请输入用户名">
  <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
  <p>用户名不能为空!</p>
</div>
</body>
</html>